The crippling web attacks on multinational companies threaten to escalate after WikiLeaks founder Julian Assange urged supporters to protect the whisteblowers' site from "instruments of US foreign policy".

Whitehall is preparing for an online backlash from the Anonymous cyber activists that brought down sites belonging to Visa, Mastercard and PayPal last week in response to the companies cutting ties with WikiLeaks. Downing Street said sites that deal with tax returns or benefits for millions of Britons are on red alert for a possible cyber attack.

Speaking though a written statement from his Wandsworth prison cell, Assange last night said he was "calling for the world to protect my work and my people" from renewed pressure to restrict the site from publishing more leaked documents.

A number of companies hosting WikiLeaks' online operations, including Amazon and PayPal, have terminated their relationship with the whistleblowers' site after pressure from Joe Lieberman, chair of the US homeland security committee.

Assange is today appearing at Westminster magistrates court to appeal against the decision to refuse him bail, pending extradition to Sweden over allegations of sexual assault. He said last night that his convictions were "unfaltering" despite the extradition attempt, according to a written statement sent to Australian broadcaster the Seven Network by his mother, Christine Assange.

"We now know that Visa, Mastercard, PayPal and others are instruments of US foreign policy. It's not something we knew before," Assange said in a statement likely to add new impetus to the spate of cyber attacks being perpetrated by Anonymous in support of WikiLeaks.

A subgroup of the amorphous "hacktivists" is preparing to hack and deface US government websites with pro-WikiLeaks propaganda, according to a Sky News journalist. Anonymous has previously made explicit its ambition to bring down the websites belonging to the US Senate and Swedish government.

"We are aware of an issue affecting some online banking customers and we are working to resolve this as soon as possible," RBS said in a statement. "We apologise to affected customers for any inconvenience this has caused."